Shroud enclosed inverted surface piercing propeller outdrive

ABSTRACT

A shrouded outdrive propels a high-speed boat having a hull for high-speed passage through water. The hull has at least one bow at the forward end and at least one transom at the stem. A tubular shaft extends at a small angle (6° to 12°) from the boat transom into the water, and a drive shaft is arranged within the tubular shaft. A propeller is mounted to the drive shaft for partial immersion in the water so that a lower portion of the propeller extends into the water during high-speed floating passage of the boat and a upper portion of the propeller is above the water during high-speed floating passage of the boat. A shroud is arranged about the propeller and is disposed below the water and adjacent the propeller. A mount holds the shroud to form a shroud-enclosed channel during high-speed passage of the boat through the water in which the propeller rotates. A plate horizontal to the undisturbed passing water surface overlies the departure side of the propeller at a radial distance of about two thirds (⅔) of the radius of the propeller. This plate immediately abuts the departure blading of the propeller in the direction of boat movement through the water and assures immersion of the lower pitch departure side of the partially immersed propeller in water for more efficient propulsion. Embodiments are disclosed where the plate is utilized as the necessary support for the shroud. Additionally, both the shroud and the plate can have small angular variations with respect to the surface of the undisturbed surface through which the high-speed hull passes.

This invention relates to outdrives for boats having partially immersed surface-piercing propellers. More particularly, a high-speed boat is provided with a surface-piercing propeller enclosed within an inverted shroud which effectively defines a channel isolating the propulsion effects of the outdrive from extraneous torques common in surface-piercing propeller outdrives. Moreover, an overlying plate improves propeller performance on the departure portion of the propeller blading from the partially immersed propeller.

The outdrive disclosed herein is applicable to all planing hulls—usually proceeding at speeds in excess of 18 mph. This disclosure relates to patrol boats, yachts, mega yachts, and so-called speed boats. Regarding ski boats, it is to be understood that the outdrive herein generates a “rooster tail”, a stream of airborne elevated water propelled by the propeller immediately astern of the outdrive. For that reason, the outdrive is not generally acceptable for ski boats.

In the following discussions, the testing of the outdrive was performed at a high horsepower (4,000 hp) and at a high speed of about 160 mph with the propeller rotating at 6,000 to 7,000 rpm. However, the principles set forth herein are applicable at much lower powers and speeds so long as a partially immersed propeller is utilized with a planing hull at speeds in excess of 18 mph.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,667,415 by the present inventor discloses a surface-piercing propeller enclosed within a metal shroud. The shroud extends over the top of the surface-piercing propeller in all illustrated embodiments.

In U.S. Pat. No. 5,667,415, the water churned upwardly by the rotation of the propeller is deflected by an overlying shroud. The interaction of the overlying shroud with the blade tends to reduce the turbulence overlying the propeller. The instabilities of the boat arising from stem lift and bow immersion of the outdrive propeller are substantially reduced. Moreover, the operator finds it much easier to operate the controls of the boat since the overlying shroud acts as a partial barrier for lateral movements of the water which tend to cause the propeller to “walk” to one side of the vessel, exerting a turning force on the boat relative to the water.

The elimination of the instabilities associated with the shroud thereon utilizes the positions of the inner surfaces of the shroud. The shroud is typically far enough away from the plane of rotation of the propeller as to prevent interference by the shroud to the rotation of the propeller itself as well as the shroud being drawn into the propeller. The inner surfaces of the shroud members contribute to keeping the center shaft thrust direction stable so that there is a reduced tendency for the propeller to lift out of the water and cause the operator of the boat to fight the steering and trim gears of the boat. The propeller configuration is different from standard propeller units. The propeller is smaller in diameter with wide thick blade tips that make it very strong and efficient. This allows the boat to get on plane quickly and with ease and maintains the achieved plane even when the rpms of the system are decreased (conventional boats tend to fall off plane when this occurs).

The inventor hereof has routinely conducted extensive testing of outdrives in San Francisco Bay and elsewhere. As a result of such testing, and through careful examination of a number of test models—exceeding 100 in the last five years, several important discoveries were made as follows.

A typical outdrive trails the transom of a high-speed planing hull. The portion of the outdrive propeller below the center of rotation of the propeller is typically below the surface of and is therefore immersed in the water, presuming that the water is undisturbed. The shaft of the propeller extends from the transom downward at an angle with respect to the surface of the undisturbed water when the high-speed planing hull is on plane. This has the beneficial result of keeping most of the shaft of the outdrive out of the water. Typically, this angle is from 6° to 12°. The following examples will use an angle of 6°.

The shaft of a typical outdrive has a relatively large diameter. It includes an outer tubular housing and an inner rotating shaft to supply rotational power to the propeller. Typically, the driving shaft is supplied with two sets of bearings. A first bearing is a universal joint on the shaft, the universal joint enabling the shaft to be “steered”. The second bearing is immediate the propeller at the distal end of the shaft from the boat. Having the shaft extend from the transom of the boat, downward at an angle of 6° to 12° from the horizontal, the major part of the shaft and surrounding tubular member need not be dragged through the water. This saves considerable friction with respect to the water, and this angular disposition of outdrives is universally used.

In the following description, “working surface” refers to an arbitrarily selected portion of a propeller blade. This arbitrary “working surface” is selected by measuring radially outward of the blade of a propeller, here a 14-inch diameter propeller. The radial distance chosen herein is five inches. The measurement of the angle of the working surface is taken tangent to the rotation of the propeller.

The reason for this definition is that propeller blades have changing working blade angles from the hub of the propeller to the extremity or periphery of the blades. In the usual case, the pitch is high adjacent the hub and gradually decreases in a radially outward direction. By having a “working surface” (pitch chosen on an arbitrary radial tangent to the direction of propeller rotation), it is possible to generate a convenient working definition of propeller pitch angle with respect to the shaft. Using this definition, some of the working principles of this invention are more easily understood.

It was discovered that the 6° downward orientation of the outdrive shaft has the effect of producing variable pitch propeller blading on opposite sides of the partially immersed propeller. Specifically, this may be seen by taking a representative “working surface” on the surface of a propeller. Say on a 14-inch diameter propeller, this chosen “working surface” happens to be in the middle of a propeller blade at a radial distance of five inches from the center of rotation of a propeller having a seven-inch radius (or 14-inch diameter). Placing a level device along the “working surface” tangent to the direction of propeller rotation and measuring the angle of the “working surface” with respect to the outdrive shaft will yield a constant angle of the working surface with respect to the shaft. For example, this angle can be 54°. At any position of rotation of the “working surface” with respect to the shaft, this angle will always be the same (e.g. 54°) with respect to a plane including the axis of the drive shaft of the propeller.

But everyone forgets that the propeller shaft itself is at an angle. Say that angle is 6° with respect to the horizontal when the boat is planing at high speed. It has been discovered that this produces variable propeller pitch on opposite horizontal sides of the propeller. As these variable propeller pitches are integral to the shrouding that is placed around the improved outdrive, the characteristics of the variable pitches must be understood.

As is well known, most single propellers rotate counterclockwise following the well-known “right-hand rule”. By extending the right-hand thumb in the direction of the propeller shaft, the fingers when naturally curled give the direction of rotation of the propeller. When two propellers are used, one propeller rotates counterclockwise and the other propeller clockwise. And since both types of propellers are always a possibility in an outdrive propeller, the working surfaces of the propeller entering the water and the working surfaces of the propeller leaving the water are referred to, regardless of whether the propeller rotates in the right- or left-hand direction.

As will be shortly developed, the entry pitch of the working surface (angle of attack with respect to the passing undisturbed water) is increased upon entry into the water by the angle of the shaft with respect to the water. Similarly, the departure pitch of the working surface is decreased upon departure from the water by the angle of the shaft with respect to the water. This discovery plays an important part in the design described below.

Consider, for example, the case of the entry pitch of the working surface. As was previously developed, the working surface has a 54° angle with respect to the propeller shaft. But the propeller shaft is inclined at 6°. Adding this 6° to 54°, the angle of attack of the entry pitch of the working surface with respect to the undisturbed water through which the propeller passes upon entry into the undisturbed water level now becomes 60°.

The same observation applies to the departure pitch of the working surface. Again the working surface has a 54° angle with respect to the propeller shaft. But the propeller shaft is inclined at 6°. Subtracting this 6° from 54°, the angle of attack of the entry pitch of the working surface with respect to the undisturbed water through which the propeller blade passes upon departure from the undisturbed water level now becomes 48°.

The important thing to understand is that with an outdrive having a shaft inclined from the horizontal by a small angle (here 6°), the entry pitch of any working surface on a blade is higher than the departure pitch of any working surface on the blade by the value of the shaft inclination.

If rapid acceleration and high propeller output power are wanted, low pitches on propellers are desirable. For example, tugboat propellers have low pitch so that large vessels may be slowly moved. Similarly, sailboat auxiliary propellers have low pitch so that the boats may maneuver in adverse weather conditions (i.e. keeping off the rocks in heavy weather). Low-pitch propellers are not intended for high speed.

If one wants high speed, high pitches are desirable. For example, racing boat propellers have high pitch so that the racing boat can proceed at high speed. High-pitch propellers are not intended for low speed.

It is further important to appreciate the practical effect of the pitch change in a partially immersed outdrive propeller. The entry half of the propeller has higher pitch than the departure half of the propeller. As a result, at low speed and upon acceleration, the departure pitch will be more ideal. Upon reaching higher speed, the entry pitch of the propeller will be more ideal.

The propellers for use in the outdrive of the present invention rotate at high power and high speed; for example all of the applicable testing for this invention has been accomplished in a twin hull boat having a 4000 Hp Lycoming Gas Turbine Engine with propeller rotating at between about 6,000 to 7,000 rpms. Propellers having mechanically variable pitches are not practical.

The testing of this outdrive should not be confused with the minimal conditions that are necessary to make the outdrive operable. Any planing hull proceeding at more than 18 mph will suffice. Further, the power expended to do this can be relatively low. All that is needed is sufficient power to make the boat hull plane.

Having discussed this discovery of the variable pitch of an outdrive propeller, discoveries concerning the disturbance of water by a propeller proceeding through the water at high speed now become relevant. When a boat proceeds at high speed—say 160 mph, standing water is disturbed before the blade of the propeller passes through it. In other words, there is a disturbance in advance of blade entry to the surface of the water. There is a well-known disturbance after the blade passes through the water; any person standing at the stern of a propeller-driven vessel and observing its wake can observe this disturbance. It is not well known, however, that disturbance occurs in the direction of boat travel in advance of the passage of the propeller blades through the water.

First, it may well be that shock waves travel through water faster than the high-speed (e.g. 160 mph) passage of the boat.

Second, the variable pitch phenomena encountered with outdrives also has an effect as follows.

If a propeller is pulled through the water without rotation, the “windage” of the propeller will cause the propeller to rotate. This is a well-known phenomena for sailors repairing large engines at sea on ships underway. Specifically, the shaft of the engine being repaired must be locked, and the ship must be moved at low speed to maintain steerage, otherwise the windage of the propeller will cause the engine under repair to rotate, creating an extraordinarily dangerous condition.

When the propeller is rotated at a speed which is “neutral” to the rate of the passing water, other than displacement effects, the propeller will neither have windage nor a propulsive force.

In the usual case, the propeller is rotated to propel water at a considerably faster speed than actual passage of the boat through the water. The propeller has slippage with respect to the passing water that is essential to its propelling effect. Anyone who has observed the wake of a propeller-driven ship is familiar with this result.

In the outdrive of this invention, the entry side of the propeller has a higher pitch, driving the water at higher speed. The departure side of the propeller has lower pitch, driving the water at lower speed. Both pitches will considerably exceed the rate of passage of the boat through the water. For example, if the boat is proceeding through the water at 160 mph, both the entry high-pitch side of the propeller and the departure low-pitch side of the propeller will drive water at speeds exceeding the 160 mph speed of the boat.

There is a further surprising effect. When the entry side of the propeller is compared to the departure side of the propeller, water build-up in advance of the departure side of the propeller will be more pronounced than water build-up in advance of the entry side of the propeller.

The reason for this water build-up differential is directly related to the variable pitch between the departure and entry sides of the propeller. Specifically, since the departure side has lower pitch and moves water at the propeller more slowly, water build-up in advance of the departure of the partially immersed propeller blade will be greater. Similarly, since the entry side has higher pitch and moves water at the propeller more quickly, water build-up in advance of the entry of the partially immersed propeller blade will be lesser. The present invention uses the greater build-up of water on the departure side of the propeller to its advantage. Specifically, a horizontal barrier is placed at approximately two-thirds (⅔) of the propeller radius directly overlying the departure side of the partially immersed propeller. This has the effect of keeping the low-pitch departure side of the propeller immersed in water for more efficient propulsion.

Plates overlying propellers used in the prior art are known. So-called “cavitation” plates are an example. These plates, used for example over outboard propellers, prevent water “flashing” into steam (cavitation). As distinguished from the plate of the present invention, these plates are arranged over an entirely immersed propeller.

Plates on outdrives have also been used on shrouds or fins, these plates being arranged over the upper two-thirds (⅔) of a propeller. However, these plates were parallel to the shaft, and never parallel to the plane of the undisturbed water. These plates have the effect of directing reverse water jets at and over the transom of the boat to which they are attached, especially during coming up to speed or decelerating from speed.

Further, the plates have been separated by several inches (typically in the order of three to four inches) in advance of the propeller. Plates with this spacing cannot cooperate with the accumulation of water in advance of the departure side of the propeller. Water in the gap between the propeller and plate is not controlled and cannot provide the improved propulsion of the present invention.

It has further been discovered that inversion of the shroud from the preferred embodiments shown in U.S. Pat. No. 5,667,415 of the inventor produces superior results. Specifically, an inverted or “upside down” shroud is used. The inverted shroud defines an enclosed operating channel for the surface-piercing portion of the propeller which isolates the partially immersed propeller from imparting unwanted torques to high-speed hulls driven by the disclosed outdrive. Stem uplift with bow immersion is avoided. Further, crawling or “helm” exerted to one or the other side of the boat is substantially reduced.

The “upside down” shroud renders the direction of propeller rotation essentially irrelevant, as it forms a separate and isolated chamber from the remainder of the water that the boat is passing through. For example, whether a so-called “right-hand propeller” or a “left-hand propeller” is utilized is irrelevant. Further, the slope of the wake where propeller immersion occurs is not as important. The disclosed shroud has the effect of isolating what might otherwise be undesired torques on the vessel propelled by the outdrive.

BRIEF SUMMARY OF THE INVENTION

A shrouded outdrive propels a high-speed boat having a hull for high-speed passage through water. The hull has at least one bow at the forward end and at least one transom at the stem. A tubular propeller shaft extends at a small angle (6° to 12°) from the boat transom into the water with a shaft within the tubular propeller shaft. A propeller is mounted on the shaft for partial immersion in the water so that a lower portion of the propeller passes below and into the water during high-speed floating passage of the boat, and an upper portion of the propeller passes above the water during high-speed floating passage of the boat. A shroud is arranged about the propeller with the shroud being disposed below the water and adjacent the propeller. A mount for the shroud holds the shroud around the propeller so that the propeller operates within a shroud-enclosed channel during high-speed passage of the boat through the water. A plate horizontal to the undisturbed passing water surface overlies the departure side of the propeller at a radial distance of about two-thirds (⅔) of the radius of the propeller. This plate immediately abuts the departure blading of the propeller and assures immersion of the lower pitch departure side of the partially immersed propeller in water for more efficient propulsion. Embodiments are disclosed where the plate is utilized as the necessary support for the shroud. Additionally, both the shroud and the plate can have small angular variations with respect to the surface of the undisturbed surface through which the high-speed hull passes.

An advantage of the inverted shroud is that it effectively defines a channel in the water in which the partially immersed propeller can operate. Forces tending to cause the partially immersed propeller to “walk” or steer the boat by causing “helm” (steering bias) are controlled. Specifically, the shroud-created channel isolates the outdrive from reacting with the water to either side of the propeller.

An additional advantage of the inverted shroud is that it provides a smooth acceleration of the watercraft to cruising speed. It is not accompanied by propeller spinning at high speed with propeller cavitation to the surrounding water. Further, at low planing speeds, the outdrive tends to maintain planing and does not allow the driven hull to “fall” off of the plane and into the water in a displacement mode.

Further, the inverted shroud can itself be adjusted in pitch, either with the angle of the outdrive or independent of the angle of the outdrive. This adjustment in pitch of the shroud can trim lifting forces on the hull of the high-speed boat being propelled by the outdrive. In the usual case, adjustments in shroud trim will be made to avoid undue stem lift and reactive pressure pushing the bow of the high-speed boat into the water.

An advantage of the plate overlying the departure side of the partially immersed outdrive propeller blading is that it confines water over the departure blading at a level well above the “undisturbed” water line. Propeller blades, departing a plane above the normal water line, pass through a layer of water that is elevated above the plane of where the water would be if it was undisturbed. In such passage, it is possible for the “lower pitch” departure blading to exert a propelling effect on the water.

An advantage of this propelling effect of the low-pitch portion of the departure propeller blading is two-fold. First, this portion of the blading accounts for the superior acceleration characteristics of this outdrive design. When the boat is accelerating, the low pitch of the departure blading apparently adds acceleration. This acceleration is extraordinarily high over many comparable designs that have been tested.

Second, even when the boat is at full (high) speed, the “low-pitch” portion of the propeller is much more efficiently utilized. Being that the low-pitch portion of the propeller has an increased “dwell time” in the passing water, the propulsion contribution of the low-pitch departure blading is increased by the overlying plate.

It will also be understood that this overlying plate operates parallel to the surface of the undisturbed water. Slight angles of inclination (much less than the 6° to 12° inclination of the propeller shaft) can be applied to the plate. These angles of inclination will be independent of the shaft and the shroud and again can be used to fine-tune forces tending to either lift or depress the outdrive at the stem of the boat.

A further advantage of both the plate and the inverted shroud is that it provides the propeller with protection. While debris can conceivably be introduced into the interstices between the propeller, plate and inverted shroud, in the usual case debris will be deflected. In most cases, debris not deflected will be pulverized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of the boat illustrated in FIG. 1 of U.S. Pat. No. 5,667,415 entitled “Marine Outdrive with Surface Piercing Propeller and Stabilizing Shroud”, this boat now being fitted with the outdrive of this invention;

FIG. 2 is a schematic perspective view of an outdrive illustrating propeller blading and a working surface of that propeller blading relative to the inclined outdrive shaft, the entry portion of the propeller blading relative to the undisturbed water, and the departure portion of the propeller blading relative to the undisturbed water, with the increased water level on the departure portion of the propeller blading being schematically shown;

FIG. 3A is a perspective taken looking toward the transom of a boat having an outdrive according to this invention illustrating the mounting with a flat plate, five-bladed propeller, and hydraulic cylinder support for steering the outdrive;

FIG. 3B is an end elevation of a propeller with an underlying shroud shown in FIG. 3A showing the propeller with the departing blades raising the water level in advance of the passage of the propeller with the overlying plate parallel to the surface of the water confining the water below the departing blades to enable efficient drive from the departing blade side of the propeller;

FIG. 3C is a side elevation of FIG. 3B illustrating the immediate proximity of the plate terminating adjacent the edge of departing blades of the propeller;

FIG. 4 is a view of the outdrive of FIGS. 3A, 3B and 3C illustrating independent angular adjustment of the shroud relative to the rest of the outdrive;

FIG. 5A shows an embodiment of the outdrive with the inverted shroud omitted and only the plate producing the improved propulsion of this invention;

FIG. 5B illustrates the inverted shroud with a rectilinear profile; and

FIG. 5C illustrates the inverted shroud with one side curved and the opposite side linear.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a high-speed planing hull H having a transom T has an outdrive O. Hull H passes over water having an upper surface 10. Outdrive O has a partially immersed propeller P surrounded by a shroud S which extends below, around and adjacent the propeller.

Referring further to FIG. 1, it is important to note the angle between the plane of upper surface 10 of the water and the centerline 14 of outdrive O shaft. Specifically, outdrive O has an angle of 6° with respect to upper surface 10. This angle can vary over a wide range, from 3° to 12°. In a narrower range, this angle can be from 4° to 9°. Here it is illustrated at the preferred angle of about 6°. Further, these angles are taken when the hull H is underway in a planing disposition at air speeds in the range of 30 mph to 160 mph. Air speeds above 160 mph should be avoided because of the danger of hull H becoming airborne.

Hull H is on the order of 50 feet in length with a displacement of 8,000 pounds. It is driven by a Lycoming gas turbine engine outputting 1,250 hp. At speeds approaching 160 mph, propeller P turns at speeds in the range of 6,000 to 7,000 rpms. Propeller P is typically of a modified construction, such as the 22-inch propeller manufactured by Rolla SP Propellers SA of Balerna, Switzerland. Thereafter, for the application here, the blades are truncated so that they are about 14 inches in diameter. Relative to conventional outdrives, the blading here illustrated is truncated; the propeller shape is accurately represented in the attached drawings.

Brief reference will now be made to FIGS. 3A, 3B and 3C. Referring to FIG. 3C, hull H is shown with outdrive 0 protruding from transom T. A tubular propeller shaft 20 has an inner drive shaft 22. Drive shaft 22 extends between a universal joint 24 adjacent transom T and a propeller bearing 26 adjacent propeller P. Drive shaft 22 is coaxial with centerline 14.

FIGS. 3A and 3B illustrate the steering and adjustment of outdrive O relative to water. Hydraulic steering cylinders 30 are illustrated with transom T being omitted. Specifically, port steering cylinder 31, center cylinder 32, and starboard steering cylinder 33 are illustrated. Since drive shaft 22 is on universal joint 24, by using hydraulic steering cylinder 30, both the adjustment of outdrive O in angle to water surface 14 and side-to-side steering angle can easily occur. Since the propeller and steering are essentially in the prior art, they are not further described herein.

Having set forth the general configuration, attention now can be turned to FIG. 2 for explaining the variation of the propeller pitch with respect to the propeller P.

Outdrive propeller P is typically immersed below the surface 10 of the water from the center of rotation 30 of the propeller to immerse just the lower half of the propeller in the water, presuming that the water is undisturbed. Shaft 22 of the propeller extends from the transom downward at a 6° angle with respect to surface 10 of the undisturbed water when the high-speed planing hull is on plane. This has the beneficial result of keeping most of the shafts 20, 22 of the outdrive out of the water. Typically, this angle can be from 6° to 12°. Six degrees will be used in the following examples.

The shaft of a typical outdrive normally has a large diameter, here approximately five inches. It includes an outer tubular housing 20 and an inner rotating shaft 22 to supply rotational power to propeller P. Having the shaft extend from the transom of the boat, downward at an angle of 6° to 12° from the horizontal, the major part of the shaft and surrounding tubular member is kept from having to be dragged through the water. This saves considerable friction with respect to the water, and this angular disposition of outdrives is universally used.

It is preferred to use a 22-inch Rolla Propeller manufactured by Rolla SP Propellers SA of Balema, Switzerland. The blade is truncated so that the original 22-inch diameter ends up being 15 inches. The propeller can be generically described as a “cleaver-style” propeller. While other propellers will do, this propeller constitutes the presently preferred design.

In the following description, the definition “working surface” will be used to describe an arbitrarily selected portion of a propeller blade. This arbitrary “working surface” 30 is selected by measuring radially outward of the blade of a propeller, here a 15-inch diameter propeller. The chosen radial distance is five inches. The angle of the working surface tangent to the rotation of the propeller is measured with respect to the plane of the upper surface 10 of the water.

There is a reason for this arbitrary definition. Propeller blades have changing working blade angles from the hub of the propeller to the extremity of the blades. In the usual case, the pitch is high adjacent the hub and gradually decreases as that pitch is measured radially outward. By having a “working surface” 30 (pitch chosen on an arbitrary radial tangent to the direction of propeller rotation), it is possible to generate a convenient working definition of propeller pitch in angle with respect to the shaft. Using this definition, some of the working principles of this invention can be more easily described.

It has been discovered that the 6° downward orientation of the outdrive shaft has the effect of producing variable pitch propeller blading on opposite sides of the partially immersed propeller. This may be seen by taking a representative “working surface” 30 on the surface of a propeller. Say on a 14-inch diameter propeller this chosen “working surface” 30 happens to be in the middle of a propeller blade at a distance of five inches of radius from the center of rotation of a propeller having a seven-inch radius (or 14-inch diameter). Placing a level device along the “working surface” tangent to the direction of propeller rotation and measuring the angle of the “working surface” with respect to the outdrive shaft will yield a constant angle of the working surface with respect to the shaft. Say for example this angle is 54°. So at any position of rotation of the “working surface” 30 with respect to the shaft, this angle will always be the same, that is, 54°, with respect to a plane including the axis of the drive shaft of the propeller.

However, the propeller shaft itself is at an angle. That angle is illustrated here at 6° with respect to the plane of the undisturbed water when the boat is planing at high speed. This produces variable propeller pitch on opposite horizontal sides of the propeller. As these variable propeller pitches are integral to the shrouding that is placed around the improved outdrive of the present invention, the variable pitches must be understood.

As is well known, most single propellers rotate counterclockwise following the well-known “right-hand rule”. By extending the right-hand thumb in the direction of the propeller shaft, the fingers when naturally curled give the direction of rotation of the propeller. Thus it will be understood that FIG. 2 illustrates the more common right-hand propeller.

Where two propellers are used, one propeller rotates counterclockwise and the other propeller clockwise. And since both types of propellers are always a possibility in an outdrive propeller, reference will be made to working surfaces 30 of the propeller entering the water and working surfaces 30 of the propeller leaving the water, regardless of whether the propeller has right- or left-hand rotation.

Thus, the entry pitch of the working surface (angle of attack with respect to the plane of the passing undisturbed water) is increased upon entry into the water by the angle of the shaft with respect to the water. Similarly, the departure pitch of the working surface is decreased upon departure from the water by the angle of the shaft with respect to the water. This discovery is an important consideration in the design that follows.

Referring to FIG. 2, consider the case of the entry pitch of the working surface 30, this entry working surface 30 being toward the viewer in the perspective view of FIG. 2. The working surface has a 54° angle with respect to a plane including the propeller shaft. But the propeller shaft is inclined at 6°. Adding this 6° to 54°, the angle of attack of the entry pitch of the working surface with respect to the undisturbed water though which the propeller passes upon entry into the undisturbed water level now becomes 60°. This is illustrated in FIG. 2.

Consider the case of the departure pitch of the working surface 30. This working surface 32 is away from the viewer in the perspective view of FIG. 2. Again the working surface has a 54° angle with respect to the propeller shaft. But the propeller shaft is inclined at 6. Subtracting this 6° from 54°, the angle of attack of the entry pitch of the working surface with respect to the undisturbed water through which the propeller blade passes upon departure from the undisturbed water level now becomes 48°.

The important thing to recognize is that with an outdrive having a shaft inclined from the horizontal by a small angle (here 6°), the entry pitch of any working surface on a blade is higher that the departure pitch of any working surface on the blade by the value of the shaft inclination.

This has the following practical effect. The entry half 35 of propeller P has higher pitch than the departure half 36 of the propeller. As a result, at low speed and upon acceleration, the departure pitch of departure half 36 will be more ideal. Upon reaching higher speed, the entry pitch of the entry half 35 of propeller P will be more ideal.

Based on this discovery of the variable pitch of an outdrive propeller, the disturbance of water by a propeller proceeding through the water at high speed becomes relevant. In summary, it has been discovered that when a boat is proceeding at high speed—say 160 mph, standing water is disturbed before the blade of the propeller passes through the standing water. In other Words, there is a disturbance in advance of blade entry to the surface of the water. There is a well-known disturbance after the blade passes through the water; any person standing at the stem of a propeller-driven vessel and observing its wake recognizes this disturbance. It is not well known, however, that disturbance also occurs in the direction of boat travel in advance of the passage of the propeller blades through the water.

First, it may well be that shock waves transmit in water faster than the high-speed (e.g. 160 mph) passage of the boat.

Second, the variable pitch phenomena related to outdrives also has an effect as follows.

In the usual case, the propeller is rotated to propel water at a considerably faster speed than actual passage of the boat through the water. The propeller has slippage with respect to the passing water that is essential to its propelling effect. Anyone who has observed the wake of a propeller-driven ship is familiar with this result.

On the outdrive of this invention, the entry side of the propeller has a higher pitch, driving the water at higher speed. The departure side of the propeller has lower pitch, driving the water at lower speed. Both pitches will considerably exceed the speed of the boat through the water. For example, where the boat is proceeding through the water at 160 mph, both the entry high-pitch side 35 of the propeller and the departure low-pitch side 36 of the propeller will drive water at speeds exceeding the speed of the boat.

There is a further surprising effect. When the entry side of the propeller is compared to the departure side of the propeller, water build-up in advance of the departure side of the propeller is more pronounced than water build-up in advance of the entry side of the propeller. This is illustrated by a surface build-up resulting in an elevated waterline surface 10 a shown with respect to departure half 36. From this illustration, it will be understood that the drive passes from left to right of the illustrated perspective. It can further be seen that this build-up is well in advance of propeller P.

The reason for this water build-up differential is directly related to the variable pitch between the departure and entry sides of the propeller. Specifically, since the departure side has lower pitch and moves water at the propeller more slowly, water build-up in advance of the departure of the partially immersed propeller blade will be greater. Similarly, since the entry side has higher pitch and moves water at the propeller more quickly, water build-up in advance of the entry of the partially immersed propeller blade will be lesser.

As will hereafter be understood with respect to FIGS. 3A, 3B and 3C, the greater build-up of water on the departure side of the propeller can be advantageously used.

FIG. 3A illustrates in perspective a view of the new shrouded outdrive O of the present invention. Propeller P has bracket 42 that overlies cylindrical propeller shaft 20. Bracket 42 supports a flat plate 40 immediately before propeller P. The underside of plate 40 is roughly parallel with the plane of the upper surface of the undisturbed surface of water through which outdrive O passes. Plate 40 is above the plane of upper surface 10 of the water.

Regarding this elevated placement of the lower surface of plate 40, a horizontal barrier is placed at approximately two-thirds (⅔) of the propeller radius, and it directly overlies the departure side of the partially immersed propeller. This has the effect of keeping the low-pitch departure side of the propeller immersed in water for more efficient propulsion.

This effect is best understood by returning to FIG. 2. At the departure half 36 of propeller P, waterline 10 a rises in advance of the passage of outdrive O through the undisturbed water. This rise occurs until the bottom surface of plate 40 is encountered, which confines the water to below the surface of plate 40.

Returning to FIG. 3C, another important aspect of plate 40 is apparent. Plate 40 terminates immediately ahead of the leading edge of propeller P, and the distance between them is kept as small as practical. The separation needs only be sufficient to assure that the trailing edge of plate 40 and the leading blade edges of propeller P do not physically interfere and that normal handling of the outdrive O does not bend or deflect either the propeller P or the plate 40 so as to cause interference.

It is important to note that plate 40 has a beneficial effect primarily on the departure side 36 of propeller P; plate 40 has no appreciable effect and is not required on entrance side 35 of plate 40. Here, however, plate 40 is part of mount 42 holding shroud S around propeller P. Thus, in the illustrated embodiment plate 40 is symmetrical.

Returning to FIGS. 3A and 3B, shroud S is mounted at the side-to-side extensions 44 from plate 40. Shroud S is invert and arcuate; it extends below, around and about propeller P. For purposes of boat control, shroud S includes skeg 50. Skeg 50 supplements the action of shroud S and maintains outdrive O on course through the water without applying torques to the boat steering.

Shroud S, being invert and arcuate, extends below, around and adjacent partially submersed propeller P and thereby defines a channel in the water as outdrive O passes through that water at high speed. Shroud S prevents water circulation to the side of propeller P and assures that propeller P only drives water fore and aft of outdrive O. The disposition of a shroud under propeller P is not shown in U.S. Pat. No. 5,667,415.

Referring to FIG. 4, shroud S and plate 40 are pivotal about an axis 60 overlying propeller P (obscured from view). Hydraulic cylinder 63 extends between a first clevis 61 on cylinder 32 and a second clevis 62 on plate 40. In this way small adjustments can be made to the angle of plate 40 and shroud S. It is to be noted that for purposes of understanding, a relatively great deflection in angle of plate 40 and shroud S is shown; in actual fact this deflection can be quite small. In the usual case it is utilized to apply trim from the outdrive to the hull, for example by preventing the stem from being unduly lifted due to lift applied at the stem.

FIG. 5A shows plate 40 functioning to keep the outgoing blading immersed in water for a greater dwell time in its total rotational cycle. This improves propulsion. It is preferred to use a truncated shroud S for this embodiment that does not surround propeller P. In other words, plate 40 will be operable in the absence of a surrounding shroud S.

Referring to FIG. 5B, it is emphasized that the inverted shroud S can be other than a smooth arc. For example, the shroud S is shown with angles of 100° utilized in squaring the rear elevation of the propeller.

Referring to FIG. 5C, an inverted shroud S is shown having a curvilinear starboard side with a linear port side. The curvilinear starboard side enables outgoing propeller blading to cooperate with shroud S in raising the water to plate 40.

As the foregoing demonstrates, there are two principal aspects to the present invention. The configuration of plate 40 and shroud S can be varied. So long as plate 40 creates additional dwell time of the departing blades within a passing water stream, the above-described function of plate 40 will be practiced. Further, so long as shroud S provides an isolated channel for operation of the outdrive without extraneous torques being introduced to the propelled hull, this aspect of the invention will be practiced. 

1-18. (canceled)
 19. In a marine outdrive for a boat, the boat having a transom, a propeller shaft extending downward at an angle from the horizontal with respect to the transom, a propeller having a center of rotation on the shaft, a shroud arranged about the propeller, and a mount extending from the boat for the shroud holding the shroud about the propeller, the improvement to the mount and shroud comprising: the mount holding the shroud so that the shroud is disposed below, around and adjacent the propeller and forms a channel below, around and adjacent the propeller to isolate water flow within the channel from water flow to the sides of the channel.
 20. The improvement in a marine outdrive for a boat according to claim 1 wherein: the mount includes a flat plate mounted at a distance from and overlying the center of rotation of the propeller shaft, an upper portion of the propeller rotating above a lower surface of the plate; and the plate having a boarder terminating immediately adjacent to propeller blades departing from a waterline taken relative to the boat so that water caused to accumulate by departing propeller vanes is confined to below the plate.
 21. The improvement in a marine outdrive for a boat according to claim 2 wherein the plate is independently adjustable in angle with respect to the propeller.
 22. The improvement in a marine outdrive for a boat according to claim 1 wherein the shroud is independently adjustable in angle with respect to the propeller.
 23. In a marine outdrive for a boat, the boat having a transom, a propeller shaft, a propeller having a center of rotation on the shaft, a shroud disposed about the propeller, and a mount extending from the boat for the shroud holding the shroud about the propeller, the improvement to the mount and shroud comprising: the mount including a flat plate mounted at a distance overlying the center of rotation of the propeller shaft with a departing portion of the propeller rotating above a lower surface of the plate; and the plate terminating immediately adjacent to departing propeller plates from a waterline taken relative to the boat so that water caused to accumulate by the departing propeller vanes accumulates and is confined to a space below the plate.
 24. The improvement in a marine outdrive according to claim 5 wherein the mount holds the shroud and so that the shroud is disposed below, around and adjacent the propeller to form a channel extending below, around and adjacent the propeller and in a direction of movement of the boat through the water to isolate water flow within the channel from water flow outside of the channel.
 25. The improvement in a marine outdrive according to claim 5 wherein the plate is independently adjustable in angle with respect to the propeller.
 26. The improvement in a marine outdrive according to claim 6 wherein the shroud is independently adjustable in angle with respect to the propeller.
 27. A marine outdrive for mounting to a boat transom, the boat having a hull for high-speed floating passage on the surface of water, the marine outdrive comprising: a tubular propeller shaft extending from the boat transom into the water, a shaft within the tubular propeller shaft, a propeller mounted to the shaft for partial immersion in the water so that a lower portion of the propeller extends into the water during high-speed floating passage of the boat and an upper portion of the propeller is located above the water during high-speed floating passage of the boat, a shroud in an invert arcuate configuration having a curved portion that is disposed in the water and adjacent the propeller, and a mount for the shroud holding the shroud around, under and about the propeller so that the propeller operates within a shroud-enclosed channel during high-speed passage of the boat through the water.
 28. The marine outdrive for mounting to a boat transom according to claim 9 wherein: the mount includes a flat plate overlying and arranged at a distance from the center of rotation of the propeller shaft, a portion of the propeller rotating above the lower surface of the plate; and the plate terminates immediately adjacent to propeller vanes departing from a waterline taken relative to the boat whereby the departing propeller vanes cause an accumulation of water in front of the departing propeller vanes which is confined to a space below the plate.
 29. The marine outdrive for mounting to a boat transom according to claim 9 and a first angle adjustment device for independently controlling an angle of the plate relative to the propeller.
 30. The marine outdrive for mounting to a boat transom according to claim 10 including a second angle adjustment device for independently controlling an angle of the plate relative to the propeller.
 31. A marine outdrive for mounting to a boat transom, the boat having a hull for high-speed floating passage on the surface of water, the marine outdrive comprising: a tubular propeller shaft extending from the boat transom into the water, a shaft within the tubular propeller shaft, a propeller mounted to the shaft for partial immersion in the water so that a lower portion of the propeller extends into the water during high-speed floating passage of the boat and an upper portion of the propeller is located above the water during high-speed floating passage of the boat, a shroud extending into the water and arranged adjacent the propeller, and a flat plate overlying a center of rotation of the propeller shaft and arranged at a distance from the propeller shaft, an upper portion of the propeller rotating above the lower surface of the plate, the plate terminating immediately adjacent to-propeller vanes departing from a waterline in the direction in which the boat moves through the water so that water accumulating in front of the departing propeller vanes accumulates in a space below the plate.
 32. The marine outdrive for mounting to a boat transom according to claim 13 wherein the shroud extends about, around and below the propeller.
 33. The marine outdrive for mounting to a boat transom according to claim 13 including an angle adjustment device for independently controlling an angle of the plate relative to the propeller.
 34. The marine outdrive for mounting to a boat transom according to claim 14 including an angle adjustment mechanism for independently controlling an angle of the shroud relative to the propeller.
 35. A high-speed boat comprising in combination: a hull for high-speed passage through water, the hull having at least one bow at a forward end and at least one transom at a stem of the hull, a tubular propeller shaft extending from the boat transom into the water, a shaft within the tubular propeller shaft, a propeller mounted to the shaft for partial immersion in the water so that a lower portion of the propeller extends into the water during high-speed floating passage of the boat through the water and an upper portion of the propeller is located above the water during high-speed floating passage of the boat through the water, a shroud in an invert arcuate configuration having a curved portion disposed in the water about, around and below and closely adjacent the propeller, and a mount for the shroud holding the shroud around the propeller so that the propeller operates within a shroud-enclosed channel during high-speed passage of the boat through the water.
 36. A high-speed boat comprising in combination: a hull for high-speed passage through water, the hull having at least one bow at a forward end and at least one transom at the stem, a tubular propeller shaft extending from the boat transom into the water, a shaft within the tubular propeller shaft, a propeller mounted to the shaft for partial immersion in the water so that a lower portion of the propeller extends into the water during high-speed floating passage of the boat through the water and an upper portion of the propeller is located above the water during high-speed floating passage of the boat, a shroud adjacent the propeller, and a flat plate overlying a center of rotation of the propeller shaft and spaced a distance above the propeller shaft, a portion of the propeller rotating above a lower surface of the plate; and the plate terminating in the direction of movement of the boat through water immediately adjacent propeller vanes so that water accumulating in front of the rotating propeller vanes is confined to a space below the plate. 